In general, the magnet used for the NMR apparatus is constituted by a coaxial nest type multi-layer solenoid. The magnet is placed in such a state that the center axis of the magnet points to the vertical direction. A port for inserting a sample to be measured, which is a through hole in the vertical direction, is provided in the proximity of the center axis of the magnet. The sample is inserted into the port from a position on the upper side, and the probe enclosing an antenna (a detection coil) for detecting a signal is inserted into the port from a position on the lower side.
The sensitivity of detection of an NMR signal varies in dependence on the shapes of the sample and the antenna as well as a positional relation between the sample and the antenna. The sensitivity can be increased by, for example, placing a solenoid-type antenna in a direction perpendicular to a main magnetic field generated by the magnet and placing the sample at a position penetrating the antenna. It's described by pp. 325–326 of “NMR Descriptions” by Youji Arata published by Maruzen in 2,000.
In the conventional NMR, however, the sample cannot be placed perpendicularly to the main magnetic field except for a special application such as a microprobe in which a solenoid-type antenna is wound directly around an extremely small test tube containing the sample. Therefore, placing the sample by such a way is not general.
In order to meet such a problem, a superconductivity magnet apparatus has been proposed configuring the magnet into a split-type, and providing an insertion hole for the sample at a side of the magnet so that the sample can be inserted by utilizing a gap between split magnets. For example it's described in Japanese Patent Laid-open No. H7(1995)-240310.
In general, a strong-magnetic-field superconductivity magnet is made of a compound superconductivity material requiring a high-temperature heat-treatment process, such as Nb3Sn and Nb3Al or the like. For this reason, coil bobbins are made of stainless steel, such as SUS316 or SUS316L, having a good heat resistance. However, even the magnetism of SUS316 or SUS316L, which is generally said to be non-magnetic, cannot be ignored in an NMR magnet, and the uniformity of the magnetic field deteriorates due to SUS bobbins.
The coil structure in the conventional NMR magnet is axis-symmetrical. To be more specific, the coil structure is symmetrical on the basis of an axis of the magnetic field or an axis of the coil. Thus, an error magnetic field generated by bobbins is an axis-symmetrical component. With regard to the axis-symmetrical error magnetic field, if a main coil is designed so as to compensate the error magnetic field in advance, no problem will be raised. However, in the case of a split-type magnet allowing the sample to be inserted from the side face of the magnet, due to a cutout of the bobbin for providing bore (through hole), an axis-unsymmetrical error magnetic field, namely an error magnetic field not symmetrical on the basis of an axis, is generated. The NMR magnet cannot compensate the axis-unsymmetrical error magnetic field by merely designing the above main coil.
In order to compensate the error magnetic field caused by a manufacturing error of the magnet, a shim coil group is provided on the NMR magnet. In the NMR magnet, there is also contained a coil for compensating the axis-unsymmetrical error magnetic field. However, the magnitude of the error magnetic field caused by the non-symmetry of the magnet is generally greater than the magnitude of the error magnetic field caused by a manufacturing error of the magnet. In consequence, the size of the coil for compensating the magnetic field considerably increases.
Concerning a superconductivity magnet apparatus, which has a port allowing an access to the center of the magnetic field from the direction other than the axial direction of the magnet, the present invention is intended to generate a uniform magnetic field, without noticeably increasing a magnetic-field compensation power of a magnetic-field compensation means such as a magnetic-field compensation coil.